1. Field of the Invention
The present invention is related to a non-contact tag antenna which communicates RFID reader/writer.
2. Description of the Related Arts
A system which enables a reader/writer to read information from a tag by transmitting a signal of approximately 1 W from the reader/writer, receiving this signal at the tag-end, and returning a response signal to the reader/writer, again, using the UHF band (860 to 960 MHz) radio signals, is called an RFID system. Although the communication distance thereof differs according to the tag antenna gain, chip operation voltage, and peripheral environment, it is about 3 m. A tag comprises an antenna with a thickness of 10 to 30 μm and an LSI chip which is connected to the antenna feed point.
FIGS. 1A to 1C are diagrams explaining the tag antenna used in a conventional RFID system. FIG. 2 is a diagram showing an equivalent circuit of an RFID tag antenna. FIG. 3 is a diagram showing an analysis example according to an admittance chart of a conventional tag antenna.
As shown in FIG. 2, the LSI chip can be equivalently represented by the parallel connection of resistance Rc (for example, 1200Ω) and capacitance Cc (for example, 0.7 pF) . This is shown in the position indicated by a circle in FIG. 3, in the admittance chart. On the other hand, the antenna can be equivalently represented by the parallel connection of resistance Rc (for example, 1000Ω) and inductance (for example, 40 nH). By parallel-connecting both LSI chip and antenna, the capacitance and inductance resonate, and as seen from the equation f0=1/(2π√{square root over ( )}(LC)), of the resonant frequency, the antenna and chip can be matched with the desired resonant frequency f0, and the reception power at the antenna is sufficiently supplied to the chip-end.
As a basic antenna used as a tag antenna, a dipole antenna of a total length of 145 mm, shown in FIG. 1A, is considered. In this antenna, a dipole part 10 is connected to a feeding part 11, electric power is extracted from a signal received by the dipole part 10, the feeding part feeds the chip and transfers the signal per se to the chip, as well. However, as indicated by a triangle in FIG. 3, if f=953 MHz, Ra=72Ω and the imaginary part=0. However, because an extremely high value of about 1000Ω is required for the radiation resistance Ra of the RFID tag antenna, Ra must be increased. Therefore, it is well-known that a folded dipole antenna with a total length of about 145 mm, as shown in FIG. 1B, is implemented and the Ra can be increased to about 300Ω to 1500Ω, depending on line width. Aside from the dipole part 10 in FIG. 1A becoming a folded dipole part 10a, FIG. 1B is the same as FIG. 1A. FIG. 3 shows an example of Ra=1000Ω. Furthermore, as shown in FIG. 1C, by connecting the inductance part 12 in parallel to this folded dipole antenna, it is rotated to the left in the admittance chart and has an imaginary component (Ba=−1/ωLa) of an absolute value which is the same as the chip (Bc=−ωCc). The shorter the inductance length is, the smaller the La value and larger the rotation amount. In this way, the imaginary component Bc of the chip and imaginary component Ba of the antenna are the same magnitude, are cancelled, and resonate.
This imaginary component cancellation is the most important factor in RFID tag antenna design. On the other hand, although it is preferable that the resistance Rc of the chip and the radiation resistance Ra of the antenna match, it is not necessary for these to match exactly, and antenna reception power can be supplied to the chip without any problems if their ratio is about two or less.
The foregoing describes the basic design method for RFID tag antennas, and it is necessary to design the basic antenna such that Ra=1000Ω at the point where the design frequency f=953 MHz and the imaginary part=0 and an inductance (Ba=−1/ωLa; La=40 nH) which has the same absolute value as the susceptance (Bc=ωCc; Cc=0.7 pF) of the chip is connected in parallel.
Refer to Non-Patent Reference 1 with regards to dipole antenna.
Non-Patent Reference 1: The Institute of Electronics, Information and Communication Engineers. Antenna Kougaku Handbook (Antenna Engineering Handbook). Ohmsha, Ltd. ISBN 4-274-02677-9
However, because an antenna with a height of about 15 mm and width of about 145 mm is too large and impractical, miniaturization is necessary. For example, an antenna which has been miniaturized to about a half or a quarter of a card size (86 mm×54 mm) is more practical. However, when the antenna is miniaturized, resonance conditions do not match with the chip to be resonated therewith because the resonance frequency, which has an imaginary part=0, increases in inverse proportion to the miniaturization of the antenna if the antenna is designed by the foregoing design method.